In general, a reluctance machine is an electric machine in which torque is produced by the tendency of a movable part to move into a position where the inductance of an energized phase winding is maximized. In one type of reluctance machine, the energization of the phase windings occurs at a controlled frequency. These machines are generally referred to as synchronous reluctance machines. In another type of reluctance machine, circuitry is provided for detecting the position of the movable part (generally referred to as a "rotor") and energizing the phase windings as a function of the rotor's position. These types of machines are generally known as switched reluctance machines. The present invention is applicable to both synchronous and switched reluctance machines.
The general theory of the design and operation of reluctance machines in general, and of switched reluctance machines in particular, is known in the art and is discussed, for example, in Stephenson and Blake, "The Characteristics, Design and Applications of Switched Reluctance Motors and Drives", Presented at the PCIM '93 Conference and Exhibition at Nuremberg, Germany, Jun. 21-24, 1993.
Conventional reluctance machines include only one set of main phase windings that are selectively energized to produce output torque in a desired direction. Typically, a rotor position transducer ("RPT") is used to detect the angular position of the rotor with respect to the stator. In such systems, the RPT will provide rotor position information to a controller that controls the energization of the switched reluctance machine.
The use of only one set of phase windings is believed to limit the maximum output torque for conventional machines. Moreover, the use of only one set of phase windings often results in undesirable variations in output torque ("torque ripple") that occur when the current is commutated from one phase winding to another phase winding.
A further limitation of conventional switched reluctance machines is that they require an RPT element for proper control purposes. The costs associated with such RPTs often place switched reluctance motors at a disadvantage, especially in applications where such motors compete with open-loop induction motors which do not require RPTs. While some have proposed "sensorless" techniques for detecting the magnitude of the back-EMF of an unenergized phase winding in a reluctance machine and commutating the phase windings when the magnitude reaches a certain level, such techniques are often limited by, and impractical because of the relatively low back-EMFs induced in the unenergized phase windings associated with many reluctance machines.
It is an object of the present invention to overcome these and other limitations of conventional reluctance machines by, inter alia, providing an improved reluctance machine system that more fully utilizes the iron and the self and mutual inductances of the phase windings of the machine for positive torque output to obtain higher torque output with minimal losses; that provides for a smoother torque output with minimal torque ripple; that provides a cost-effective and efficient system for torque production; and that allows for efficient sensorless operation of a switched reluctance machine.
Other objects and advantages of the present invention will be apparent to one of ordinary skill in the art having the benefit of this disclosure.